Luhnow mentioned on the pre-game radio spot yesterday that Singleton would probably come up after the AAA playoffs. Interestingly, he also said Matt Duffy might. Duffy, of course, was just named MVP of the PCL. He's never really cracked the prospect lists, partly due to age (he turned 26 back in February).
This year, Duffy had 20 HR and 104 RBI in 127 games, all with Fresno, putting up a .294/.366/.484 line. His career slash line is very similar. He's mostly been a 3B, but has played a fair amount of 1B too.
Maybe a callup would be primarily about rewarding him for 5 years of being a good org player and capping it with the MVP this year. Maybe they see him as a RH bench weapon (he did hit LHP better than RHP, this year at least). He started the year off slowly but in both July and August hit over .300 with an OPS near 1.000.